The present invention relates to a method of producing a record carrier, the record carrier itself and a method of producing signals from such a record carrier.
In United States patent application Ser. No. 798,709 filed Feb. 12, 1969, by Gerhard Dickopp, Hans-Joachim Klemp, Horst Redlich and Eduard Schuller and now Patent No. 3,652,809, there is disclosed a system for reproducing signals recorded on a record carrier whose surface is provided with undulations corresponding to the variation of the signals with respect to time, and which is provided with a playback head having a contact element exerting a contact force upon the surface of the support moving by. This system is characterized by the fact that the playback head serves as a pressure sensing pickup excited by the time variation of the contact force corresponding to the deformations, the playback head having the shape of a transducer body mechanically biased in the direction of the contact force. The contact takes place either directly or via an essentially inherently rigid coupling piece. Preferably the elastic deformation of the surface of the carrier due to the contact force is considerably greater than the opposed lateral deflection of the contact element of the playback head due to the compression.
A carrier which yields elastically under the pressure of a pickup producing a high tracking force, was in direct contrast with the view then held by those skilled in the art that to raise as much as possible the upper frequency limit of the reproduction band, the record support should be as hard as possible and the pickup should be sprung as softly, i.e., as compliant as possible. The publication "Factors Affecting the Stylus/Groove Relationship in Phonograph Playback Systems" by G.R. Bastiaans in the Journal of the Audio Engineering Society, October 1967, Vol. 15, No. 4, pages 389 to 399 is a suitable example of this unfruitful conventional view with regard to the extension of the reproduction range towards higher frequencies. That publication contains a detailed exposition of the theory of these relationships and gives values of the upper limiting frequencies which can be reached for the kind of phonograph record play-back employed up to then.
A further coherent frequency range up to several MHz which, according to the results of experiments, offers the possibility for practical recording and reproduction and which is above the upper limiting frequency, regarded as insuperable, of the range which could be utilized with conventional means, was opened up by the method disclosed in the afore-mentioned application. That method employs a pick-up having a skid-shaped portion in engagement with the surface of the record and experiencing only a very small amplitude of movement while the undulations on the surface of the record containing the signal, on the other hand, experience considerably greater compressions. Thus, the possibility is afforded of recording and reproducing a broadband signal which may serve, for example, for the storage and reproduction of a television transmission. In this case, as mentioned, use is made of a skid-shaped pickup whose leading edge gradually approaches the surface of the record while its trailing edge is relatively steep so that the deformations rapidly pass out of range of contact with the pickup during the playback whereas they "creep" into the range of contact at the leading edge.
A frequency-modulated carrier frequency oscillation is generally recorded such that the amplitude of its undulations remain unaltered and all hills and dales of the undulations recorded are identical so that the skid-shaped portion of the pickup, which always covers several wavelengths of the carrier oscillation, also comes into contact with all hills, without one hill or the other being omitted, as could occur in amplitude modulation with hills of different heights. In this manner, the pickup records as a pressure pulse the departure of each individual hill out of its contact range with the "sharp" pickup trailing edge. The above description of the method according to the above-identified patent application is presented herein primarily because there is not yet any printed publication available about this system of so-called "pressure playback".
In the recently-developed method of the type described above because of the broad coherent working frequency band with an upper cut-off frequency of several MHz, a number of narrow-band signals can be stored by the known multi-channel technique, or a broadband signal, for example a video signal, which occupies the entire frequency range available, can be recorded and reproduced. In the latter case, the need may arise to accommodate one or more further signals, for example a sound signal or color signal, in the same recording.
The principle of pressure playback of a frequency-modulated carrier wave, which has been outlined broadly, appears at first to exclude the superimposition of an additional signal oscillation or modulated carrier wave, because the advantage of equal heights in the oscillation to be played back would be lost by such a superimposition. As a result of superimposing the beat of two signals to be recorded, conditions resembling amplitude modulation with different positions of the hills would possibly result so that the shaped pickup might no longer detect the undulation hills situated in dips in the beat curve.